The present invention pertains to a control system for automatic adjustment of the amount of light supplied to a target from a light source, and more particularly to a control system of the type incorporating a motor for positioning a shutter. The invention is especially adapted for use with a video camera used in medical inspections.
Cameras using high intensity light sources are desirable because of their natural color output and find application in surgical inspection devices. One difficulty with cameras using high intensity light sources in surgical inspection is controlling the amount of light supplied from the light source and reflected from the target to the camera. The amount of light reflected from the target is a function of the target distance, that is the distance from light guides used with the video camera to the target, and the reflectance of the target. Controlling the amount of light reflected from the target is difficult as the target distance and the reflectance of the target change as the surgical inspection device moves through the body.
A known system for surgical inspection includes a high intensity light source and a shutter for controlling the amount of light supplied from the light source, which is applied to a first light guide in order to illuminate a target. A second light guide passes light from the target to a video camera, which senses the amount of light reflected from the target in order to adjust the shutter position. Although this system provides automatic control of the light supplied to the target according to the amount of light reflected from the target, the system has certain undesirable characteristics and features. The known system utilizes a shutter sensor to sense the amount of light passing through the shutter and another sensor to sense the light level of the signal output from the video camera. The shutter light sensor of this known system presents several difficulties. The shutter light sensor is expensive and difficult to obtain. Additionally, the known systems include six calibration potentiometers, and thus requires a significant amount of calibration time when the system is initially set up. Furthermore, the system must be recalibrated each time the light source is replaced. Because the lamps have a relatively short life expectancy (approximately 250 hours), a large amount of time is spent recalibrating the system. A further difficulty in using the prior art systems is that the sensors must be mechanically aligned as part of the calibration procedure.
An additional consideration in providing a motorized control for a shutter is the response time of the system. The response time of the known system is a function of the individual time constants of the sensors and the motor. As a result, the known system is slow in responding to changes in the target distance and reflectance, and thus the system is often supplying an improper amount of light to the target during dynamic situations.